Dimmable lighting circuits typically include a phase-cut dimmer, in which either the leading edge or the trailing edge of an alternating current (AC) input supply is cut, usually by means of a thyristor or a triac. For conventional incandescence lighting systems, the optical output of the light-bulb responds only slowly to changes in the electrical input, since the output is determined by the temperature of the filaments and thus changes as the filaments cools. Apart from extremely low dimming levels (that is to say when the current is supplied from the thyristor or triac for only a very small part of the AC half cycle, which typically operates at 50 Hz or 60 Hz), the filament does not cool appreciably during the phase-cut part of the cycle, and thus a user is not aware of the phase cutting.
A preferred configuration of LED lighting circuits, to provide good current control even at very low dimming levels, comprises a constant current source in series with the LEDs. The current provided by the current source is controlled in dependence on the position of the phase cut dimmer. A power converter is used which, together with a capacitor on the output, smoothes the power to the LEDs in order to provide power even during the “cut” part of the AC phase. It will be appreciated that the current source need be “constant” with over timescales of interest. In particular very high frequency fluctuations, of the order of microseconds or milliseconds, may not be of relevance, provided that such fluctuations do not results in drift or fluctuations over longer timescales such as tens of milliseconds.
In contrast to incandescent lighting systems, the optical output of the LED light sources in LED lighting systems changes very quickly with changes to the electrical input. The optical flux output of an LED changes within microseconds of a change in the electrical current. Thus any instabilities or perturbations in the electrical current through an LED are reflected in its optical output. The optical response of a user is not fast enough to be aware of changes occurring at speeds of microseconds, and so if instabilities of perturbations can be corrected sufficiently quickly, they will not be visible. However, fluctuations lasting several tens, or hundreds of milliseconds or longer may become visible. It would be desirable to reduce or even eliminate such slow fluctuations.